Seismic streamers used in marine exploration include a streamer cable towed behind a survey vessel. External cable-leveling devices and position-determining sensors, such as acoustic transducer pods, are coupled to the streamer at various positions along its length. The cable-leveling devices, known as birds, include motor-controlled vanes, or wings, which set the depth of the cable.
Typical mechanisms for coupling birds and acoustic pods to a streamer include collars attached at selected positions along the length of the streamer. Each collar comprises cylindrical inner and outer races surrounding the streamer. The inner race is affixed to the streamer. The outer race, to which the bird or acoustic pod is coupled, can freely rotate about the inner race and, hence, the streamer. The outer race, or locking collar, has a keyhole-shaped locking slot designed to slidably receive a dovetail pin extending from a chamber in a pylon on the bird or pod. The dovetail pin has a flat top tapering on opposite sides toward a narrow neck. The neck extends from a broader cylindrical body. The neck is narrow enough to permit the pin to slide along the locking slot in the locking collar. The pylon is attached to the locking collar by inserting the top of the extended dovetail pin through the circular opening in the locking slot and sliding the neck of the dovetail pin along the slot and away from the circular opening. As the pin slides to the end of the locking slot opposite the circular opening, a locking rod, under spring pressure in a chamber in the pylon, pops into the circular opening of the locking slot, thereby preventing the dovetail pin from sliding back and releasing the engagement.
An eccentric pin extending transversely from one side of the pylon to the other has a central camming surface that engages the domed bottom of the dovetail pin. The eccentric pin terminates in screwdriver- or wrench-adjustable drive surfaces, such as slotted hexagonal adjustment heads, external to the pylon. As the eccentric pin is rotated, the camming surface forces the dovetail pin between extended and retracted positions. With the dovetail pin retracted and the locking rod extended, the pylon is held fast to the collar. The adjustment head has a small indented reference mark to indicate the position of the eccentric pin.
Studs, attached to the spring-loaded locking rod and protruding through slots on opposite sides of the pylon, act as levers for manually retracting the locking rod from its locked position in the circular openings of the locking slot.
A latching mechanism manufactured by Syntron, Inc. of Houston, Tex. and shown in its U.S. Pat. No. 4,290,124 to Cole operates as just described. A similar mechanism, using a locking screw instead of a locking pin, is described in Syntron's U.S. Pat. No. 3,931,608, also to Cole. In both Syntron locking mechanisms, the locking pin or screw is separately adjustable from the locking rod. A third mechanism, manufactured by DigiCOURSE, Inc. of Harahan, La., a wholly-owned subsidiary of the assignee of this invention, is similar to that hereinbefore described, but differs in that the locking pin and the locking rod are coupled together by an internal linkage so that loosening (extending) the locking pin also retracts the locking rod, and vice versa. In the DigiCOURSE mechanism, there are no studs on the locking rod, adjustment being by means of the adjustment heads. A fourth mechanism, having a different mode of operation from the previous three and from the improved mechanism of this invention, is the subject of U.S. Pat. No. 4,879,719 to Dumestre. The Dumestre mechanism includes a lever-actuated cylinder with a cam extending through a locking slot. The lever is used to rotate the cylinder and its cam between insertable and locked orientations.
In the case of a cable-leveling bird, two similar latching mechanisms spaced about a foot from each other are used to fasten the bird pylon to the collar--one near the front of the bird, the other in the vicinity of the bird's wings. Depending on the orientation of the wings, access to the latching mechanism near the wings may be obstructed. With the Syntron mechanism, it is difficult to disengage the locking rod to release the bird because access by hand to the studs is awkward whenever the wings are more or less horizontal. With the DigiCOURSE mechanism, although it has no hand-maneuverable studs to disengage the locking rod, visual access to the reference mark on the locking mechanism's adjustment heads is impaired. Visual access is important because it provides a clear indication that the locking mechanism is, in fact, latched. An improperly latched mechanism can result in the costly loss of a bird at sea and the even more costly disruption of the seismic data-gathering process. For example, grit or other debris contaminating the locking rod's chamber in the pylon can prevent the spring from forcing the rod into its extended, locked position. Without an easily viewed visual indication that the locking rod is fully extended, it is more likely that improperly latched birds are deployed. Another problem is that debris in the water can bump into the studs of the locking rod, forcing the rod to disengage from its locked position during deployment.
Thus, an object of the invention is to prevent cable-leveling birds and other collar-attached devices from falling off seismic streamer cables during deployment or servicing.
The invention further provides advantages such as a single adjustment point for both the dovetail pin and the locking rod, a clearly visible indication of locked and unlocked conditions to increase operator awareness of the existing condition, and positive stops signifying complete engagement and disengagement of the locking rod.